Informatica

This paper studies the generic construction of certificate-based signature (CBS) from certificateless signature (CLS). This paper proposes a new generic conversion from CLS to CBS which is more intuitive, simpler, and provably secure without random oracles than the current one. To develop the security proof, we put forth one novel CLS security model which features a previously neglected but nontrivial attack and hence captures the CLS security notion more comprehensively. We show that many existing CLS schemes can be proved secure in the current model by slightly modifying its original security proof. Following this conversion, many provably secure CBS schemes can be constructed from the corresponding existing CLS schemes.

A proxy signature scheme enables an original signer to delegate its signing capability to a proxy signer and then the proxy signer can sign a message on behalf of the original signer. Recently, in order to eliminate the use of certificates in certified public key cryptography and the key-escrow problem in identity-based cryptography, the notion of certificateless public key cryptography was introduced. In this paper, we first present a security model for certificateless proxy signature schemes, and then propose an efficient construction based on bilinear pairings. The security of the proposed scheme can be proved to be equivalent to the computational Diffie–Hellman problem in the random oracle with a tight reduction.

In 2008, based on the two-party Diffie–Hellman technique, Biswas proposed a contributory group key exchange protocol called the Group-DH protocol. This contributory property is an important one of group key agreement. Unfortunately, in this paper we show that the proposed Group-DH protocol is not a contributory group key exchange protocol. Therefore, we propose an improved group key exchange protocol with verifiably contributory property based on the same Diffie–Hellman technique. When an identical group key is constructed, each participant can confirm that his/her contribution is actually included in the group key. We show that the improved protocol is provably secure against passive attacks under the decisional Diffie–Hellman assumption. As compared to the previously proposed group key exchange protocols, our protocol provides contributiveness and the required computational cost is suitable for low-power participants in a network environment.

In this paper, we propose a new ID-based threshold signature scheme from the bilinear pairings, which is provably secure in the random oracle model under the bilinear Diffie–Hellman assumption. Our scheme adopts the approach that the private key associated with an identity rather than the master key of PKG is shared. Comparing to the-state-of-art work by Baek and Zheng, our scheme has the following advantages. (1) The round-complexity of the threshold signing protocol is optimal. Namely, during the signing procedure, each party broadcasts only one message. (2) The communication channel is optimal. Namely, during the threshold signing procedure, the broadcast channel among signers is enough. No private channel between any two signing parties is needed. (3) Our scheme is much more efficient than the Baek and Zheng scheme in term of computation, since we try our best to avoid using bilinear pairings. Indeed, the private key of an identity is indirectly distributed by sharing a number xID∈ $\mathbb{Z}^{*}_{q}$, which is much more efficient than directly sharing the element in the bilinear group. And the major computationally expensive operation called distributed key generation protocol based on the bilinear map is avoided. (4) At last, the proactive security can be easily added to our scheme.

We revisit the password-based group key exchange protocol due to Lee et al. (2004), which carries a claimed proof of security in the Bresson et al. model under the intractability of the Decisional Diffie–Hellman problem (DDH) and Computational Diffie–Hellman (CDH) problem. We reveal a previously unpublished flaw in the protocol and its proof, whereby we demonstrate that the protocol violates the definition of security in the model. To provide a better insight into the protocol and proof failures, we present a fixed protocol. We hope our analysis will enable similar mistakes to be avoided in the future. We also revisit protocol 4 of Song and Kim (2000), and reveal a previously unpublished flaw in the protocol (i.e., a reflection attack).